Proposal: ls-lisp.el handle --group-directories-first flag

[Arthur Miller]

Just a minor 'out-of-the-box' quality of life improvement mostly for
MS Windows users. ls-lisp.el is default on MS Windows, and some users
might share their init configuration between their Gnu/Linux and MS
Windows systems, it might be nice to have ls-lisp.el recognize
`--group-directories-first' flag of Gnu ls program.

I have just piggy-back on existing code here; I am not sure if I can
actually set `ls-lisp-dirs-firs' var to `t' when
`--group-directories-first' is found, so I am just converting this flag
to `y' so we can per-use existing machinery. But I would happily skip
`y' if it is acceptable to change users choice for `ls-lisp-dirs-first'
variable when `--group-directories-first' is present.

If this would be acceptable, I can also add some notice to docs and
news.

ls-lisp.patch
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